For Aristotle, the exercise of specific behaviors in natural bodies is teleological. It is for the sake of an end. The process in which human beings acquire reason in is an example. Human beings acquire reason for the sake of becoming like the unmovable first mover. Reason makes the human existence like the divine existence.
The Teleological Framework
In the Timaeus, a divine craftsman and maker (the "demiurge" (δημιουργὸς)) arranges things so that everything in the cosmos is as good as possible. In particular, the divine craftsman ensures that human beings have the sense of sight so that through exercise of this ability they can transform themselves and make themselves better by making their minds more orderly. Human beings have the sense of sight because seeing is part of a process that gives rise to the love of wisdom and that ultimately makes human understanding take on "the invariability belonging to god."
"Let us conclude our discussion of the auxiliary causes that gave our eyes the power they now possess. We must next speak of that supremely beneficial function for which the god gave them to us. As my account has it, our sight has proved to be a source of supreme benefit to us, in that none of our present statements about the universe could ever have been made if we had never seen any stars, sun or heaven. The ability to see the periods of day-and-night, of months and of years, of equinoxes and solstices, has led to the invention of number, and has given us the idea of time and opened the path to inquiry into the nature of the universe. These pursuits have given us the love of wisdom (φιλοσοφίας), a gift from the god to the mortal race whose value neither has been nor ever will be surpassed. This is the supreme good our eyesight offers us. ... The god invented sight and gave it to us so that we might observe the orbits of intelligence in the universe and apply them to the revolutions of our own understanding. For there is a kinship between them, even though our revolutions are disturbed, whereas the universal orbits are undisturbed. So once we have come to know them and to share their ability to make correct calculations according to nature, we should stabilize the straying revolutions within ourselves by imitating the completely unstraying revolutions of the god" (Timaeus 46e-47c).
The Unmovable First Mover
Aristotle's disagreement with Plato is over the model for teleology, not the teleology itself.
What Aristotle rejects is the divine maker that Plato seems to have in mind in the Timaeus. Aristotle argues that there can be no first or last moment of time, that time is a function of motion, and that the ultimate cause of this everlasting motion must itself be completely free from change. This understanding seems to rule out the craftsmanship model for teleology in nature.
"It is impossible that movement should come into being or cease to be; for it must always have existed. Nor can time come into being or cease to be; for there could not be a before or after if time did not exist. It follows that movement is continuous in the way time is; for time is the same thing as movement or an attribute of movement. And there is no continuous movement except movement in place, and of movement in place only that which is circular is continuous. ... If something acts, this will not be enough if its substance is potentiality; for there will not be eternal movement; for that which is potentially may possibly not be. There must be a starting point (ἀρχὴν) whose substance (οὐσία) is actuality (ἐνέργεια)" (Metaphysics XII.7.1071b). "There is, then, something which is always moved in an unceasing motion, which is motion in a circle; and this is plain not in theory only but in fact. The first heaven (πρῶτος οὐρανός) must be eternal, and there is something which moves it. And since that which is moved and moves is intermediate, there is a mover which moves without being moved, being eternal, substance, actuality (ἀΐδιον καὶ οὐσία καὶ ἐνέργεια)" (Metaphysics XII.7.1072a).
Aristotle's idea is obscure, but he seems to think that the sphere of fixed stars continuously and eternally moves in a circle around the earth and that all other motion and change depends on the motion of this outermost sphere in the universe. (The fixed stars appear to rise and set. The wandering stars are the planets, which seem to change their position relative to the fixed stars.) There are unmoved movers for each of the planetary spheres below the sphere of the fixed stars. These unmoved movers move without themselves moving. They are teleological causes of motion and change. There is an unmoved mover for the sphere of the fixed stars, and this unmoved mover is one in number (Metaphysics XII.8.1074a). It is the unmovable first mover. The order in the world derives from it.
Specific behavior in natural bodies provides an example of how the order in the world depends on the unmovable first mover. Natural bodies have their specific behavior (the behavior that belongs to individuals as members of a species) for the sake of becoming like the unmovable first mover. Specific behavior makes their existence like that of the unmovable first mover.
The Divine Existence of the Unmovable First Mover
"This is the starting point (ἀρχῆς) on which the heavens and nature depend. Its life is like the best which we temporarily enjoy. It must be in that state always, which for us is impossible... Holding in actuality (ἐνεργεῖ) is the intellect (νοῦς) the divine possesses, and contemplation (θεωρία) is that which is most pleasant and best. ... If, then, the state God (ὁ θεὸς) always enjoys is as great as that which we enjoy sometimes, it is marvelous; and if it is greater, this is still more marvelous. Nevertheless it is so. Moreover, life belongs to God. For the actuality (ἐνέργεια) of intellect (νοῦ) is life, and God is that actuality; and the essential actuality of God is life most good and eternal. We hold, then, that God is a living being, eternal, most good" (Metaphysics XII.7.1072b).